Field
The disclosed concept pertains generally to voltage sensing devices, and, more particularly, to a high impedance voltage sensing device for measuring voltages that employs a virtual ground sensing circuit.
Background Information
Measuring AC voltage at a remote location in, for example, a power distribution system, can result in significant errors, especially when a high impedance voltage sensor is used. In particular, when a cable is used to transport the signal, the capacitance of the cable can lead to erroneous measurements. If a shielded cable is used (which is often the case), then the capacitance of the signal wire to the shield and the high impedance sensor form a voltage divider that results in significant signal attenuation. In addition if the voltage sensor is resistive, then the capacitive reactance introduces a significant phase shift in the signal as well. If there are other conductors in the cable, then in addition to contributing to attenuation and phase shift, there will also be crosstalk. The attenuation, phase shift, and crosstalk are all a function of the cable capacitance, which increases with cable length and varies between cable types. Furthermore, wire to wire capacitance varies within a given multi-conductor cable because the distance between all the conductors in the cable varies. Consider, for example, that in a twenty-eight conductor circular cable, the center conductors are adjacent to each other such that the wire to wire capacitance will be relatively high, whereas the center to outer conductors are spaced further apart such that the wire to wire capacitance is lower. Moreover, an AC voltage difference between conductors will result in higher capacitive currents flowing between the conductors with higher capacitances. These unintentional capacitive currents produce measurement errors. A conductor that lies between two other conductors also tends to shield those conductors from each other. The situation is further complicated when the impedance to ground differs between conductors. That is to say, all the conductors in a given cable may not be connected to high impedance voltage sensors. Rather, in a multi-purpose cable, there are likely to be other power and signal wires that have a low AC impedance to ground. This will complicate the analysis and make it all but impossible to calculate and compensate for the measurement errors.
In addition to the negative effects of parasitic capacitances, signals from high impedance sensors are also sensitive to leakage currents due to moisture in connectors. In outdoor applications, connectors are often exposed to high humidity and changing temperatures that result in condensation in the connectors. Most connectors don't provide a gas tight seal over the life of the installation, and some are directly exposed to rain water and will eventually leak. In a high voltage (e.g., 38 kV) application, the voltage sensor is often a resistive divider consisting of a 200M ohm high voltage resistor connected to a 100 k ohm low arm resistor. To introduce a 1% error, moisture would only need to reduce the surface insulation resistance of the connector system to less than 10M ohms. In practice, this happens all too often. In a recent lab experiment, a connector was placed under water and the signal dropped to 2.5% of its original value.
Another source of error is from the voltage clamp devices that are often required on both sides of the cable. On the sensor side, a TVS (transient voltage suppressor), MOV (metal oxide varistor), or zener diode may be needed to prevent the voltage from rising too high in the event that the control cable is disconnected and the low arm resistor is not present. These devices all have parasitic capacitance that results in leakage current that attenuates the signal and introduces a phase shift. The capacitance will vary with temperature, and typically is not specified in the data sheets for the parts. On the other end of the cable, the sensing circuit will also contain one of these devices. In this case, the voltage clamp is for surge protection of sensitive electronic circuits.
Furthermore, when a low level signal is to be transported over some distance (e.g., 20 to 120 feet), the first thing a designer does to minimize error is specify a shielded cable. Another technique is to use twisted pairs. Still another approach would be to use individually shielded conductors or coaxial cable. All of these alternatives increase the capacitance and can make the situation worse.